A flyback converter is a DC/DC converter with galvanic isolation between the input and the output or outputs. Specifically, the flyback converter is a buck-boost converter with the inductor split to form a transformer, having a primary side winding and secondary side windings exhibiting opposing winding directions. In a typical flyback arrangement, the secondary circuit uses a blocking diode to ensure that current flows in the secondary only when the primary switch is open.
Flyback converters with multiple outputs are known in the art. Each of the outputs is driven by a secondary winding electromagnetically interacting with the primary winding and a dedicated blocking diode. The output voltage of each secondary is provided across an output filter capacitor to a load. Variations in the output load current, voltage drops in the transformer and blocking diode, and non-ideal transformer turns-ratios require that the output voltage be actively regulated to maintain a constant level. In one embodiment, primary side regulation is used to control one of the output voltages by feeding back a control signal representative of the quantity to be controlled, and comparing the quantity to be controlled with a predetermined reference. The difference is used to control the primary side switch so as to bring the particular output in line with the predetermined reference. The remaining outputs track the regulated output as determined by the turns-ratio of the transformer; however they are not tightly regulated, particularly over a range of loads.
Primary side regulation cannot control each of the secondary outputs independently, therefore secondary side regulation is often provided. In secondary side regulation known to the prior art, also known interchangeably as secondary side post regulation, an electronically controlled switch is provided in series with the blocking diode of each of the secondaries. The switch is then controlled so as to regulate the associated secondary output voltage.
U.S. Pat. No. 6,369,525 issued Apr. 9, 2002 to Chang et al, and U.S. patent application Ser. No. 11/621,160 to Peker et al, entitled “Secondary Side Post Regulation for LED Backlighting”, filed Jan. 9, 2007 and published Jul. 12, 2007 as U.S. Patent Application Publication S/N 2007/0459421 A1, the entire contents of each of which is incorporated herein by reference, are addressed to flyback converters which provide multiple independently regulated outputs. The secondary side switches each exhibit a blocking diode in series therewith to prevent reverse current flow through the inherent body diode of the MOSFET switch. With low output voltages the forward voltage drop across the blocking diode becomes a significant factor which limits the operating efficiency of the converter.
European Patent S/N EP 0698959 A1, to Siemens, published Aug. 4, 1995, the entire contents of which is incorporated herein by reference, is addressed to a DC/DC voltage converter with a plurality of controlled secondaries. In one embodiment the electronically controlled switch comprises an inherent valve section to perform the blocking diode functionality. Unfortunately, the requirement for such an inherent valve section prevents the use of low cost MOSFET devices without providing for an additional blocking diode.
What is needed, and not provided by the prior art, is a secondary side post regulation scheme allowing for use of a MOSFET electronically controlled switch without requiring a blocking diode.